After much looking, I finally found the solution. I had specified an automatic logout interval of 60 minutes in the Security preferences panel. This automatic logout seems to conflict with the automatic sleep. When I disabled the automatic logout, all of my problems have gone away.
After sitting idle, the PowerBook can start the screen saver (set in the Desktop & Screen Saver preferences), turn off the display (set in the Energy Saver preferences), put the computer to sleep (set in the Energy Saver preferences), and/or log out the user (set in the Security preferences). There seems to be a conflict between the sleep and log out functions.

